Recent automatic transmissions or continuously variable transmissions have been demanded to be light and small and sought to be improved in power transmission capability in connection with the increased power output of the engines with which the transmissions are used in combination. The reduction in weight and size is intended to improve the fuel efficiency of the vehicles in which the transmissions are mounted.
In particular, in the case of a metal belt type continuously variable transmission, it can be reduced in size if the friction coefficient between the belt and pulleys and thus lubricating oil to be used therein is preferably an oil having properties to keep the metal-to-metal friction coefficient high.
Furthermore, the lubricating oil has also been demanded to reduce the fuel consumption. Specifically, a lubricating oil contributes to improvement in fuel economy by reducing its viscosity, stir resistance or viscous resistance upon idling of a clutch pack, or fluid film lubrication, resulting in a reduction in power loss.
A transmission fluid has been proposed, in which a friction modifier, a metallic detergent, an ashless dispersant, and an anti-wear agent are optimally added so as to retain the friction characteristics of a lock-up clutch in a good condition and provide long-lasting initial anti-shudder properties (see Patent Literatures 1 to 7 below).
For example, Patent Literature 1 discloses a transmission lubricating oil composition comprising a specific calcium salicylate, an SP-based extreme pressure additive, a specific succinimide and a boron-containing ashless dispersant, each in a specific amount, which composition exhibits excellent properties such as excellent anti-shudder properties and long-lasting fatigue life. Patent Literature 2 discloses a continuously variable transmission lubricating oil composition containing an organic acid metal salt with a specific composition, an anti-wear agent, and a boron-containing succinimide, as essential components, to have both higher metal-to metal friction coefficient and anti-shudder properties for a slip control mechanism. Patent Literature 3 discloses a long-lasting continuously variable transmission lubricating oil composition comprising calcium salicylate, a phosphorous-containing anti-wear agent, a friction modifier, and a dispersant type viscosity index improver, to have both a higher metal-to metal friction coefficient and anti-shudder properties for a slip control mechanism. Patent Literature 4 discloses a lubricating oil composition comprising a dithiocarbamate compound, a condensate of a branched fatty acid having 8 to 30 carbon atoms and amine, and an amine-based anti-oxidant, to have excellent and long-lasting anti-shudder properties. Patent Literature 5 discloses an automatic transmission fluid composition comprising calcium sulfonate, phosphorous acid esters and further a sarcosine derivative or a reaction product of a carboxylic acid and amine, to have long-lasting anti-shudder properties for a slip lock-up mechanism and long-lasting properties to prevent scratch noise in a belt type continuously variable transmission. Patent Literature 6 discloses an automatic transmission fluid composition comprising a specific alkaline earth metal sulfonate in a specific amount, which composition is excellent in oxidation stability as a fluid used for an automatic transmission with a slip control mechanism and has long-lasting anti-shudder properties. Patent Literature 7 discloses an automatic transmission fluid comprising calcium salicylate, magnesium salicylate, a specific amount of a friction modifier and a specific amount of a boric acid-modified succinimide, with excellent anti-shudder properties and a certain torque capacity.